Thermionic cathode comprising mixture of barium oxide, calcium oxide and lithium oxide

ABSTRACT

A cathode is disclosed utilizing an emission material of barium oxide, calcium oxide and lithium oxide.

Manila [451 Apr. 11, 1972 [54] THE 1'] WNIC CATHUDE (ZOMPRISING [56] References Cited MKXTU OF BARIUM OXIDE,

i I D @AMIUM OXIDE AND LIT 1 1 7 50 12/ 2 STATES PATENTS 3,0 0,5 19 @XHDE 3,076,916 2/1963 [72] Inventor: Leo J. Cronin, Watsonville, Calif. 3,250,943 5/1966 [73] Assignee: Spectra-Mat, Inc., Watsonville, Calif. [22] Filed: Nov. 18, 1970 D ds h be Primary Examineravi c on rg [21] Appl' 90,764 Assistant Examiner-Toby H. Kusmer Att0rneyLimbach, Limbach & Sutton [52] 11.8. CI ..3l3/346 DC, 252/521, 313/311,

313 34 R [57] ABSTRACT [51] h t. Cl. ..H0lj 1/14 A h de i disclosed utilizing an emission material of barium [58] Field of Search ..3l3/311, 346 R, oxide, calcium oxide and lithium oxide,

7 Claims, 3 Drawing Figures Patented April 11, 1972 I000 I000 2000 LIFE (HOURS) I000 I200 I400 FIG. 2

950 TEMPERATURE (C) TNVENTOR. LEO J. CRONIN k 2. g

ATTORNEYS 'Ill-IEIONIC CA'I'IHODE COWRISING MIXTURE F BM 01E, CALCIUM OXIDE AND r s r 1 0E The present invention is directed in general to cathodes and, more particularly, to thermionic dispenser cathodes.

Over the years various different cathode structures have been proposed with selected different materials to achieve high current densities, low evaporation and long life. Cathodes have been made in many forms. In one form, the emission material is sprayed or painted on the surface of a support member, typically nickel, tungsten or molybdenum. Ribbons, wires or screens have been added to the surface to aid in holding the emission material on the surface. Additionally, porous body structures have been utilized to slowly dispense the emission material through the pores to the emission surface. U.S. Pat. No. 2,543,728 to Lemmens et al. describes the use of a cavity to contain the emission material and a porous body portion providing communication between the cavity and the electron emission surface for migration of the emission material. U.S. Pat. No. 2,700,000 to Levi et al. describes a cathode wherein the emission material is melted into the pores of the porous-sintered body.

Barium oxide (BaO) has been the principal preferred emission material for many cathodes. However, barium oxide is extremely hygroscopic and in normal atmosphere readily converts to barium hydroxide. Consequently, many cathodes include barium oxide in a composition with another material whereby the barium oxide and/or free barium can be released for desired electron emission. Barium carbonate has frequently been used as one composition wherein the carbonate will convert to barium oxide at elevated temperatures.

In thermionic dispenser cathodes operating at higher temperatures and utilizing refractory metal bodies such as tungsten, molybdenum and the like as the porous-sintered main body portion of the cathode, barium carbonate reacts with tungsten in a manner unproductive of free barium. Consequently, a number of alkaline earth metal compositions are suggested wherein barium oxide will be held in a stable form. Materials suggested in U.S. Pat. No. 2,700,118 to Hughes et al. alkaline earth metal silicates, aluminates, thorates, berylliates, and borates. Of these materials, the barium aluminates were preferred and have been used most extensively.

Since it was known, as in Therminoic Emission from the BaO-CaO System by L. E. Grey, NATURE, Volume 165, pp. 773-774, May 13, 1950, that enhanced emission can be achieved by the substitution of calcium oxide for part of the barium oxide, barium calcium aluminate dispenser cathodes have been used. U.S. Pat. No. 3,076,916 to Koppius describes one such dispenser cathode having a porous-sintered tungsten body with an impregnate material formed of barium oxide, calcium oxide an aluminum oxide in the mole ratio of 4:1:1, respectively. U.S. Pat. No. 3,201,639 describes a similar thermionic dispenser cathode wherein the impregnate material is barium oxide, calcium oxide and aluminum oxide in the mole ratio of 5:3:2, respectively.

Broadly stated, the present invention is directed to a cathode having a metallic body and an emission material comprising a mixture of barium oxide, calcium oxide and lithium oxide.

It has been found that cathodes having the preferred compositions of the present invention produce enhanced emission over cathodes similarly constructed but having the most popular prior art emission material made up of barium oxide, calcium oxide and aluminum oxide.

Additionally, the emission material composition in accordance with the present invention has a higher melting point than that of the prior art barium calcium aluminate impregnated cathodes thereby permitting high temperature operation on the completed impregnated dispenser cathode body without deleteriously effecting the emission characteristics.

Other objects and advantages of this invention will become apparent when reading the following description referring to the accompanying drawing in which similar characters of reference represent corresponding parts in each of the several views.

In the drawings:

FIG. 1 is a side sectional view of a cathode in accordance with one embodiment of the present invention.

FIG. 2 is a graph of emission in milliamps plotted versus life for a cathode in accordance with the present invention as contrasted with a cathode in accordance with the prior art.

FIG. 3 is a graph of current in milliamps plotted versus cathode temperature for the cathode in accordance with the present invention as contrasting with a cathode in accordance with the prior art.

Referring now to FIG. 1, there is shown one embodiment of the present invention. In this embodiment of the present invention the cathode 10 includes a porous metallic body 11, of a refractory material, such as tungsten, provided with an emission material 12 substantially uniformly dispersed throughout the pores of the body for migration through the pores to an emission surface 13. The emission material 12 can be placed in the pores by mixing, pressing and sintering material 12 with metal particles such as tungsten particles when the body is of tungsten. Altemately, the cathode 10 can be formed by first forming the porous sintered body 11 by pressing and sintering tungsten particles and then melting the emission material into the pores of the body. An example of such a cathode is described below.

The body 11 1 is supported for use in a vacuum tube by means of a support cylinder 14, such as molybdenum. A heater filament 15 such as a metal wire wound in a spiral is positioned within the cylinder 14 behind the body 11 for heating the body 11 and emission material 12 for thermionic emission of electrons from the emission surface 13 when the cathode is positioned within a vacuum envelope (not shown). Cathodes in accordance with this invention are suitable for use in most vacuum tubes or gas tubes. The cathode can be used as a cold cathode wherein the heater 14 is utilized only to initiate emission or dispensed with entirely and emission derived in other ways such as by electron bombardment to produce secondary emission.

The heater filament 15 is placed in location within the cylinder 14 and potted in placed with an electrical insulating material 16 such as alumina ceramic.

The emission material 12 is a mixture of barium oxide, lithium oxide and another alkaline earth metal oxide, other than barium oxide, preferably calcium oxide. I prefer as an emission material one comprising barium oxide, calcium oxide and lithium oxide. Specific compositions of the emission material in accordance with the present invention are set forth in greater detail below and with reference to the method of formation of these materials.

I have discovered that a cathode made generally in accordance with the present invention but utilizing only barium oxide and lithium oxide does not produce satisfactory emission while an identical cathode body impregnated with an emission material of barium oxide, calcium oxide and lithium oxide not only produces satisfactory emission but greater emission than the popular barium oxide, calcium oxide and aluminum oxide type cathodes.

I have discovered that cathodes with emission materials formed of 4 moles of barium oxide, 1 mole of calcium oxide and 1 mole of lithium oxide as well as 5 moles of barium oxide, 3 moles of calcium oxide and 2 moles of lithium oxide produce greater emission and less barium evaporation than similar cathodes with emission materials of barium oxide, calcium oxide and aluminum oxide in these same mole ratios.

FIGS. 2 and 3 show graphs of emission versus life and emission versus operating temperature for cathodes in accordance with the present invention designated E" as contrasted with the more popular barium oxide, calcium oxide and aluminum oxide cathodes designated type B. The subscript l (i.e. B and E refer to cathodes where the respective materials are in the mole ratio of 4:1:1, and the subscript 2 (i.e. B andE refer to cathodes where the respective materials are in the mole ratio of 5:3:2.

While it is believed that the invention has been described thus far in sufiicient detail to enable one skilled in the art to manufacture cathodes in accordance with the present invention, a detailed operative example of a cathode of the type illustrated in Fit]. 1 will be given.

First of all, the body 11 is formed by pressing tungsten powder, approximately microns average diameter, to form an ingot which is heated in hydrogen at about 2,350 C. for approximately 20 minutes to form a porous-sintered body having a density in the range of 78-84 percent. Next, the porous body is impregnated with a filler material such as a metal or plastic and the filled ingot machined to form individual buttons for bodies having the dimensions of the desired cathodes. In the case of metal, such as copper, the tiller material can be melted into the pores, while in the case of a plastic such as methyl methacrylate, the porous body can be impregnated with the liquid plastic which then sets up and hardens in the pores. After machining, the filler material is removed by heating to elevated temperature.

For the embodiment having an emission material of barium oxide, calcium oxide and lithium oxide in the mole ratio 4:121, 4 moles barium carbonate, 1 mole calcium carbonate and 1 mole lithium carbonate are heated to a temperature of about 1,900 C. in an air atmosphere. At this elevated temperature, the carbonates convert to the respective oxides and these oxides fuse together. The resultant material is allowed to cool and harden.

For impregnating the emission material into the cathode body the cooled, hardened mass is pulverized to form a powder and heated in a hydrogen atmosphere to a temperature of about l,900 C. and brought to contact with the previously machine cathode body whereby the emission material will migrate into the pores of the body by capillary attraction. Excess emission material is removed from the emission surface.

The heater is potted" within the cylindrical support behind the cathode body by surrounding the heater with powdered alumina ceramic and heating the assembly to a temperature of about l,900 C. to sinter the alumina. With the high melting point of the emission material, it is possible to pot the heater behind the cathode without unduly exposing the cathode to high temperatures for prolonged periods as was necessary with prior art cathodes with the result that the life of the cathode structure was adversely affected.

While calcium oxide is preferred as the other alkaline earth metal to be utilized with barium oxide and lithium oxide, other alkaline earth metal oxides such as strontium oxide and magnesium oxide can be used.

The emission material can be applied to other surfaces such as by evaporation onto a metal surface from which emission is to be drawn.

Additionally the cathode can be made in the form of a directly heated cathode body as described in U.S. Pat. No. 3,514,661 to RT. Reaves assigned to the assignee of this invention.

What is claimed is:

l. A cathode comprising:

a metallic body structure of a high melting point metal and an emission material supported by said body structure,

said emission material comprising a mixture of barium oxide, calcium oxide and lithium oxide.

2. A dispenser cathode comprising:

a refractory metal body having an electron emission surface and a porous portion for migration of an emission material to said surface and emission material supported by and in contact with said body for migration through said porous portion to said surface,

said emission material comprising a mixture of barium oxide, lithium oxide and a second alkaline earth metal oxide other than barium oxide.

3. The dispenser cathode in accordance with claim 2 wherein said second alkaline earth metal oxide is calcium oxide.

4. The dispenser cathode in accordance with claim 3 wherein said barium oxide, calcium oxide and lithium oxide are in the mole ratio of about 4: 1:1, respectively.

5. A dispenser cathode comprising:

a porous high-density tungsten body with interconnecting pores communicating with an electron emission surface and an emission material located within said pores for migration through said pores to said surface,

said emission material comprising a mixture of barium oxide, lithium oxide and a second alkaline earth metal oxide other than barium oxide.

6. The dispenser cathode in accordance with claim 5 wherein said second alkaline earth metal oxide is calcium oxide.

7. The dispenser cathode in accordance with claim 6 wherein said barium oxide, calcium oxide and lithium oxide are in the mole ratio of about 4:1 1, respectively. 

2. A dispenser cathode comprising: a refractory metal body having an electron emission surface and a porous portion for migration of an emission material to said surface and emission material supported by and in contact with said body for migration through said porous portion to said surface, said emission material comprising a mixture of barium oxide, lithium oxide and a second alkaline earth metal oxide other than barium oxide.
 3. The dispenser cathode in accordance with claim 2 wherein said second alkaline earth metal oxide is calcium oxide.
 4. The dispenser cathode in accordance with claim 3 wherein said barium oxide, calcium oxide and lithium oxide are in the mole ratio of about 4:1:1, respectively.
 5. A dispenser cathode comprising: a porous high-density tungsten body with interconnecting pores communicating with an electron emission surface and an emission material located within said pores for migration through said pores to said surface, said emission material comprising a mixture of barium oxide, lithium oxide and a second alkaline earth metal oxide other than barium oxide.
 6. The dispenser cathode in accordance with claim 5 wherein said second alkaline earth metal oxide is calcium oxide.
 7. The dispenser cathode in accordance with claim 6 wherein said barium oxide, calcium oxide and lithium oxide are in the mole ratio of about 4:1:1, respectively. 